Fix fringe case (#130)

why:
 Merging [a,a] in to {[a+1,b]} caused assertion violation, doAssert
 condition was too strict.

stew/interval_set.nim

@@ -347,9 +347,9 @@ proc new[P,S](T: type Interval[P,S]; kvp: DataRef[P,S]): T =
 
 proc overlapOrLeftJoin[P,S](ds: Desc[P,S]; l, r: P): Rc[P,S] =
   ## Find and return
-  ## * either the rightmost `[l,r)` overlapping interval `[a,b)`
+  ## * either the rightmost interval `[a,b)` which overlaps `r`
   ## * or `[a,b)` with `b==l`
-  doAssert l < r
+  doAssert l <= r
   let rc = ds.leftPos.le(r) # search for `max(a) <= r`
   if rc.isOk:
     # note that `b` is the first point outside right of `[a,b)`

tests/test_interval_set.nim

@@ -195,6 +195,11 @@ suite "IntervalSet: Intervals of FancyPoint entries over FancyScalar":
         (ivVal, ivSet) = (iv, true)
       check ivVal == iv(uHigh,uHigh)
 
+    br.clear() # from blockchain sync crash
+    check br.total == 0 and br.chunks == 0
+    check br.merge(1477152,uHigh) == uHigh - 1477151
+    check br.merge(1477151,1477151) == 1
+
   test "Merge disjunct intervals on 1st set":
     br.clear()
     check br.merge(  0,  99) == 100